1. Field of the Invention
The present invention relates to an LED lamp and more particularly relates to a white LED lamp that can be used as general illumination.
2. Description of the Related Art
A light emitting diode (LED) is a semiconductor device that can radiate an emission in a bright color with high efficiency even though its size is small. The emission of an LED has an excellent monochromatic peak. To obtain white light from LEDs, a conventional LED lamp arranges red, green and blue LEDs close to each other and gets the light rays in those three different colors diffused and mixed together. An LED lamp of this type, however, easily produces color unevenness because the LED of each color has an excellent monochromatic peak. That is to say, unless the light rays emitted from the respective LEDs are mixed together uniformly, color unevenness will be produced inevitably in the resultant white light. Thus, to overcome such a color unevenness problem, an LED lamp for obtaining white light by combining a blue LED and a yellow phosphor was developed (see Japanese Patent Application Laid-Open Publication No. 10-242513 and Japanese Patent No. 2998696, for example).
According to the technique disclosed in Japanese Patent Application Laid-Open Publication No. 10-242513, white light is obtained by combining together the emission of a blue LED and the yellow emission of a yellow phosphor, which is produced when excited by the emission of the blue LED. That is to say, the white light can be obtained by using just one type of LEDs. Accordingly, the color unevenness problem, which arises when white light is produced by arranging multiple types of LEDs close together, is avoidable.
An LED lamp with a bullet-shaped appearance as disclosed in Japanese Patent No. 2998696 may have a configuration such as that illustrated in FIG. 1, for example. As shown in FIG. 1, the LED lamp 200 includes an LED chip 121, a bullet-shaped transparent housing 127 to cover the LED chip 121, and leads 122a and 122b to supply current to the LED chip 121. A cup reflector 123 for reflecting the emission of the LED chip 121 in the direction indicated by the arrow D is provided for the mount portion of the lead 122b on which the LED chip 121 is mounted. The LED chip 121 on the mount portion is encapsulated with a first resin portion 124, in which a phosphor 126 is dispersed and which is further encapsulated with a second resin portion 125. If the LED chip 121 emits a blue light ray, the phosphor 126 converts a portion of the blue light ray into a yellow light ray. As a result, the blue and yellow light rays are mixed together to produce white light.
However, the luminous flux of a single LED is too low. Accordingly, to obtain a luminous flux comparable to that of an incandescent lamp, a fluorescent lamp or any other general illumination used extensively today, an LED lamp preferably includes a plurality of LEDs that are arranged as an array. LED lamps of that type are disclosed in Japanese Patent Application Laid-Open Publications No. 2003-59332 and No. 2003-124528. A relevant prior art is also disclosed in Japanese Patent Application Laid-Open Publication No. 2004-172586.
Japanese Patent Application Laid-Open Publication No. 2004-172586 discloses an LED lamp that can overcome the color unevenness problem of the bullet-type LED lamp disclosed in Japanese Patent No. 2998696. In the bullet-type LED lamp 200 shown in FIG. 1, the first resin portion 124 is formed by filling the cup reflector 123 with a resin to encapsulate the LED chip 121 and then curing the resin. For that reason, the first resin portion 124 easily has a rugged upper surface as shown in FIG. 2. Accordingly, the thickness of the resin including the phosphor 126 loses its uniformity, thus making non-uniform the amounts of the phosphor 126 present along the optical paths E and F of multiple light rays going out of the LED chip 121 through the first resin portion 124. As a result, the unwanted color unevenness is produced.
To overcome such a problem, the LED lamp disclosed in Japanese Patent Application Laid-Open Publication No. 2004-172586 is designed such that the reflective surface of a light reflecting member (i.e., a reflector) is spaced apart from the side surface of a resin portion in which a phosphor is dispersed. FIGS. 3A and 3B are respectively a side cross-sectional view and a plan view illustrating an LED lamp as disclosed in Japanese Patent Application Laid-Open Publication No. 2004-172586. In the LED lamp 300 shown in FIGS. 3A and 3B, an LED (LED bare chip) 112 mounted on a substrate 111 is covered with a resin portion 113 in which a phosphor is dispersed. A reflector 151 with a reflective surface 151a is bonded to the substrate 111 such that the reflective surface 151a of the reflector 151 is spaced apart from the side surface of the resin portion 113. Thus, the shape of the resin portion 113 can be freely designed without being restricted by the shape of the reflective surface 151a of the reflector 151. As a result, the color unevenness can be reduced significantly.
By arranging a plurality of LED lamps having the structure shown in FIGS. 3A and 3B in columns and rows, an LED array such as that shown in FIG. 4 is obtained. In the LED lamp 300 shown in FIG. 4, the resin portions 113, each covering its associated LED chip 112, are arranged in matrix on the substrate 111, and a reflector 151, having a plurality of reflective surfaces 151a for the respective resin portions 113, is bonded onto the substrate 111. In such an arrangement, the luminous fluxes of a plurality of LEDs can be combined together. Thus, a luminous flux, comparable to that of an incandescent lamp, a fluorescent lamp or any other general illumination source that is used extensively today, can be obtained easily.
If the LED lamp 300 shown in FIG. 4 is used as general illumination, no color unevenness will be produced and a sufficiently high luminous flux can be obtained. However, the present inventors further analyzed this LED lamp 300 to discover that the LED lamp 300 with such a high luminous flux (which is sometimes called a “high-flux LED lamp”) often produces an uncomfortable glaring impression on the viewer although everybody in the prior art has been paying most of their attention to how to increase the luminous flux of the LED lamp. That is to say, as for general illumination, “the brighter, the better” policy is often too simple to work and it is not preferable to make such a glaring impression on the viewer.
According to JIS C8106, the “glare” refers to viewer's uncomfortableness or decreased ability to recognize small objects, or even every object in general, due to an inadequate luminance distribution within his or her vision, which is formed by the excessively high luminance of the luminaire within his or her sight. Generally speaking, the viewer tends to find a light source very glaring (i) if the luminance of the light source exceeds a certain limit, (ii) if the viewer's eyes have got used to the darkness surrounding him or her, (iii) if the source of the glare is too close to his or her eyes, and/or (iv) if the apparent size or the number of the glaring sources is big. Accordingly, it is believed that the viewer is very likely to find an LED lamp glaring if the LED lamp includes a plurality of LEDs, has a high luminance, and is used in a relatively dark place. Among other things, the LED lamp uses the emissions of multiple LEDs and therefore has a much stronger directivity than that of a fluorescent lamp, for example. As a result, the LED lamp tends to produce a stronger glaring impression on the viewer in many cases. Nevertheless, if the luminance of the LED lamp were decreased to reduce such a glare, then the LED lamp would be too dark to use as general illumination. Also, since the degree of that glare changes with the surroundings, there is no need to darken the LED lamp in a situation where the LED lamp should not look glaring. In view of these considerations, if there were an LED lamp that can either take anti-glare measures, or cast bright light as usual, with the glare producing conditions taken into account fully, that would be a very convenient commodity.